Y-chromosomal Adam

In human genetics, Y-chromosomal most recent common ancestor (Y-MRCA; informally also known as Y-chromosomal Adam) refers to the most recent common ancestor (MRCA) from whom all currently living people are descended patrilineally; the term Y-MRCA reflects the fact that the Y chromosomes of all currently living males are directly derived from the Y chromosome of this remote ancestor. The analogous concept of matrilinear most recent common ancestor is known as "Mitochondrial Eve" (mt-MRCA, named for the matrilinear transmission of mtDNA), the woman from whom all living humans are descended matrilineally.

By the nature of the concept of most recent common ancestors, these estimates can only represent a terminus ante quem ("limit before which") until the genome of the entire population has been examined (in this case, the genome of all living humans); thus, in 2013, the discovery of a previously unknown Y-chromosomal haplogroup was announced,[1] which resulted in a slight adjustment of the estimated age of the human Y-MRCA at between 163,900 and 260,200 years.[2]

By comparison, the age of the mt-MRCA (matrilinear MRCA) has been estimated at 140,000 to 200,000 years. By definition, it is not necessary that the Y-MRCA and the mt-MRCA should have lived at the same time,[3][4] even though current (as of 2014[update]) estimates suggest the possibility that the two individuals may well have been roughly contemporaneous (albeit with uncertainties ranging in the tens of thousands of years).[5]

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The existence of a Y-MRCA for any given human population is a necessary consequence of the assumption that every human being has a biological father, and that every human male inherits his Y chromosome from his biological father.

Although the informal name "Y-chromosomal Adam" is a reference to the biblical Adam, this should not be misconstrued as implying that the bearer of the chromosome was the only human male alive during his time.[6] His other male contemporaries also have descendants alive today, but not, by definition, through solely patrilineal descent.

Due to the definition via the "currently living" population, the identity of a MRCA, and by extension of the human Y-MRCA, is time-dependent (depends on the moment in time intended by the term "currently"). The MRCA of a population may move forward in time as archaic lineages within the population go extinct: Once a lineage has died out it is irretrievably lost and this mechanism can thus only shift the title of Y-MRCA forward in time. Such an event could be due to the total extinction of several basal haplogroups.[3] The same holds for the concepts matrilineal and patrilineal MRCAs: It follows from the definition of Y-MRCA that he had at least two sons who both have unbroken lineages that have survived to the present day. If the lineages of all but one of those sons die out, then the title of Y-MRCA shifts forward from the remaining son through his patrilineal descendants, until the first descendant is reached, who had at least two sons who both have living, patrilineal descendants. The title of Y-MRCA is therefore not permanently fixed on a single individual, and the Y-MRCA for any given population would himself have been part of a population which had its own, more remote, Y-MRCA.

In addition to the ability of the title of Y-MRCA to shift forward in time, the estimate of the Y-MRCA's DNA sequence, his position in the family tree, the time when he lived, and his place of origin, are all subject to future revisions.

The following events would change the estimate of who the individual designated Y-MRCA was:

Further sampling of Y chromosomes could uncover previously unknown divergent lineages. If this happens, Y-chromosome lineages would converge on an individual who lived further back in time.

The discovery of additional deep rooting mutations in known lineages could lead to a rearrangement of the family tree.

Revision of the Y-chromosome mutation rate (see below) can change the estimate of the time when he lived.

The time when Y-MRCA lived is determined by applying a molecular clock to human Y-chromosomes. In contrast to mitochondrial DNA, which has a short sequence of 16,000 base pairs, and mutates frequently, the Y chromosome is significantly longer at 60 million base pairs, and has a lower mutation rate. These features of the Y chromosome have slowed down the identification of its polymorphisms and as a consequence, reduced the accuracy of Y-chromosome mutation rate estimates.[7]

Methods of estimating the age of the Y-MRCA for a population of human males whose Y-chromosomes have been sequenced are based on applying the theories of molecular evolution to the Y chromosome. Unlike the autosomes, the human Y-chromosome does not recombine often with the X chromosome during meiosis, but is usually transferred intact from father to son; however it can recombine with the X chromosome in the pseudoautosomal regions at the ends of the Y chromosome. Mutations occur periodically within the Y chromosome and these mutations are passed on to males in subsequent generations. These mutations can be used as markers to identify shared patrilineal relationships. Y chromosomes that share a specific mutation are referred to as haplogroups. Y chromosomes within a specific haplogroup are assumed to share a common patrilineal ancestor who was the first to carry the defining mutation. (This assumption could be mistaken, as it is possible for the same mutation to occur more than once.) A family tree of Y chromosomes can be constructed, with the mutations serving as branching points along lineages. The Y-MRCA is positioned at the root of the family tree as the Y chromosomes of all living males are descended from his Y chromosome.

Determining the Y-MRCA's DNA sequence, and the time when he lived, involves identifying the human Y-chromosome lineages that are most divergent from each other—the lineages that share the fewest mutations with each other when compared to a non-human primate sequence in a phylogenetic tree. The common ancestor of the most divergent lineages is therefore the common ancestor of all lineages.

Current (as of 2014[update]) estimates for the age for the Y-MRCA are roughly compatible with the estimate for the emergence of anatomically modern humans some 200,000 years ago (200 kya), although there are substantial uncertainties.

Early estimates published during the 1990s ranged between roughly 200 to 300 kya,[8] Such estimates were later substantially corrected downward, as in Thomson et al. 2000,[7] which proposed an age of about 59,000. This date suggested that the Y-MRCA lived about 84,000 after his female counterpart mt-MRCA (the matrilineal most recent common ancestor), who lived 150,000–200,000 years ago.[9] This date also meant that Y-chromosomal Adam lived at a time very close to, and possibly after, the migration from Africa which is believed to have taken place 50,000–80,000 years ago. One explanation given for this discrepancy in the time depths of patrilineal vs. matrilineal lineages was that females have a better chance of reproducing than males due to the practice of polygyny. When a male individual has several wives, he has effectively prevented other males in the community from reproducing and passing on their Y chromosomes to subsequent generations. On the other hand, polygyny does not prevent most females in a community from passing on their mitochondrial DNA to subsequent generations. This differential reproductive success of males and females can lead to fewer male lineages relative to female lineages persisting into the future. These fewer male lineages are more sensitive to drift and would most likely coalesce on a more recent common ancestor. This would potentially explain the more recent dates associated with the Y-MRCA.[10][11]

The "hyper-recent" estimate of significantly below 100 kya was again corrected upward in studies of the early 2010s, which ranged at about 120 kya to 160 kya. This revision was due to the discovery of additional mutations and the rearrangement of the backbone of the Y-chromosome phylogeny following the resequencing of Haplogroup A lineages.[12] In 2013, Francalacci et al. reported the sequencing of male-specific single-nucleotide Y-chromosome polymorphisms (MSY-SNPs) from 1204 Sardinian men, which indicated an estimate of 180,000 to 200,000 years for the common origin of all human through paternal lineage.[13] or again as high as 180 to 200 kya.[14] Also in 2013, Poznik et al. reported the Y-MRCA to have lived between 120,000 and 156,000 years ago, based on genome sequencing of 69 men from 9 different populations. In addition, the same study estimated the age of Mitochondrial Eve to about 99,000 and 148,000 years.[15] As these ranges overlap for a time-range of 28,000 years (148 to 120 kya), the results of this study have been cast in terms of the possibility that "Genetic Adam and Eve may have walked on Earth at the same time" in the popular press.[5][16]

The announcement of yet another discovery of a previously unknown lineage, haplogroup A00, in 2013, resulted in another slight shift in the estimate for the age of Y-chromosomal, given by Elhaik et al. (2014) as between 163,900 and 260,200 years (95% confidence), i.e. in a range still compatible with (high) estimates made prior to the discovery of the previously unknown haplogroup.[2][17]

The revision of Y-chromosomal phylogeny since 2011 has affected estimates for the likely geographical origin of Y-MRCA as well as estimates on time depth. By the same reasoning, future discovery of presently-unknown archaic haplogroups in living people would again lead to such revisions. In particular, the presence of between 1% and 4% Neanderthal-derived DNA in Eurasian genomes implies that in the (unlikely) event of a discovery of a single living Eurasian male exhibiting a Neanderthal patrlinineal line would immediately push back TMRCA[clarification needed] to at least twice its current estimate, and questions of geographical origin would become part of the debate on Neanderthal evolution from Homo erectus.

According to Cruciani et al. 2011, the most basal lineages have been detected in West, Northwest and Central Africa, suggesting plausibility for the Y-MRCA living in the general region of "Central-Northwest Africa".[18] Scozzari et al. (2012) agreed with a plausible placement in "the north-western quadrant of the African continent" for the emergence of the A1b haplogroup [19] and the 2013 report of haplogroup A00 found among the Mbo people of western Cameroon is also compatible with this picture.[1]

^In a sample of 2204 African Y-chromosomes, 8 chromosomes belonged to either haplogroup A1b or A1a. Haplogroup A1a was identified in two Moroccan Berbers, one Fulbe and one Tuareg people from Niger. Haplogroup A1b was identified in three Bakola pygmies from Southern Cameroon and one Algerian Berber. Cruciani et al. 2011